The present invention relates to a process which can be applied in the wet screen area of existing wet-laid web-making machines with only minor equipment and procedural modifications.
In the manufacture of non-woven fibrous webs on equipment resembling paper making machines using the wet method, in which an aqueous fibre suspension containing shredded (beaten) or non-shredded natural and/or synthetic fibres, is formed into a web on a screen and subsequently dried, the fibre web is given the necessary coherence by means of either an aqueous binder composition and/or melt fibres. Such binder composition or melt fibres can be added to the fibre suspension before formation of the web or it can be applied to the already-formed water-wet web in the wet screen section of the web-making machine.
The application of the binder material to the fibres while the latter are still suspended in water involves many problems of coagulation, mixing, uniformity of association between the fibres and the binder, and fouling of the broke boxes, beaters and screens of the web-making machines and also of the water effluent from the screen area of the equipment.
The application of latex-like binder compositions to the already-formed but still wet web in the web-making machine has taken various forms including applying the compositions on the dried or partially-dried web by spraying, impregnating or printing followed by a final finishing operation. The resulting products, however, have had a harsh paper-like feel which is undesirable for many applications of non-woven webs. These known methods have also been primarily applicable to the thinner, less bulky types of webs.
German Patent Specification No. 1,221,093 describes a process in which a binding composition in the form of an aqueous foam is applied to the water-wet fibre web between the initial screening section and the first drying cylinder of the web-making machine.
The products thus made do not have the requisite smooth surface and still require the drying treatment followed by impregnating, spraying or printing the once-impregnated web with a dispersion of a synthetic polymeric material such as rubber. Such a product has a paper-like feel or hand not at all desirable for many non-woven applications.
In our British Application No. 26540/74 there is described a process in which a heat-sensitized latex is formed and applied by a knife coater or doctor blade to the non-woven web in the wet screen area of the web-making machine where the normal dewatering suction sections under the screen are relied on to draw the foam into the web. Such a procedure is an improvement over prior procedures but still does not produce the very soft hand so desired in the product and, moreover, is limited to the preparation of the thinner or light weight webs. The knife coater is troublesome to utilize due to irregularity of the web and also due to pick up of fibre on the blade and requires constant attention of operators for cleaning and adjustment in order to approximate a constant concentration of binder on the web.